In telephone installations subscriber telephones are connected to an exchange via so-called subscriber lines which are connected in the exchange to an adaption circuit, a so-called subscriber line interface circuit (SLIC). DC supply, inter alia, to the associated subscriber telephones is carried out by such a circuit.
A constant direct voltage is utilized for this power supply, according to the prior art. The current flowing through the subscriber line will thus be a function of the internal resistance of the subscriber telephone or terminal, but naturally also of the line length. The internal resistance in apparatus can be kept within narrow limits but the line length and line resistance can naturally vary considerably. If there is a constant direct voltage, this must be put at such a value that sufficient direct current can also be fed out to long lines. Thus a greater current than necessary flows through the shorter lines and larger losses than necessary have to be reckoned with in the line, as well as in tha adaption circuit, and above all in the feed resistance in this circuit.
Heat due to the losses puts a limit on the packing density of the circuits in the exchange, and it is accordingly a primary object to reduce heat generation.
It is also known to utilize a voltage source with a plurality of outputs for different voltages, the selection of voltage thus being enabled with regard to, inter alia, line length such that heat due to losses is reduced.
As a further development of such a system, a solution could be considered which utilizes a continuously variable voltage source with low self power consumption, which is controlled by the conditions on the line such that the subscriber apparatus is satisfactorily supplied with current, while the loss due to heat is minimized. A known realization according to this principle utilizes a symmetric arrangement of two so-called choppers, each feeding one of the subscriber line wires.
From the aspect of saving in components, an arrangement with only one chopper would be preferable, in spite of the unsymmetry which would be thus introduced. The Swedish Patent Application SE 78.02565-7 illustrates such an unsymmetric arrangement, in which the current feed resistance is eliminated with the exception of a small protective resistance. The magnitude of the power gain achieved is considerable, and is dependent on the relationship between the values of the simulated resistance of the apparatus and the protective resistance.